Advances in plasma processing have provided for the growth in the semiconductor industry. With the utilization of a plasma processing system, substrates may be transformed into a variety of devices, such as a micro electromechanical system (MEMS) device. Substrate may be processed in a series of operations in which materials are deposited onto a substrate surface and selectively removed (etching) from predefined areas on the substrate surface in order to form trenches, vias, and other features thereon.
Consider the situation wherein, for example, a silicon substrate having an insulator layer and a silicon layer is being etched using fluorine-based gases such as SF6, NF3, and/or CF4. The silicon layer may have a mask (such as a hard or resist mask) defining the areas available for etching. A trench may be formed during a vertical etch into the areas of the silicon layer not covered by the mask. As the silicon layer is being etched, lateral etching may unintentionally occur on either side of the sidewalls of the trench, causing one or more mask undercuts to be formed. As discussed herein, a mask undercut refers to a condition that occurs when a sidewall of a trench, vias, etc. is being undercut underneath a mask.
The aforementioned etching tends to slow down when an insulator layer is reached. One skilled in the art is aware that fluorine-based gases are less effective etchants of insulator layers, which may be a dielectric formed from organic and/or inorganic materials. Thus, when the fluorine-based etchant reaches the insulator layer, deeper lateral etching may occur at the intersection of the insulator layer and the silicon layer, causing notches to be formed in the sidewalls of the bottom trenches. As discussed herein, a notch refers to an undercut into the wall of the silicon layer near or at the insulator layer.
To facilitate discussion, FIG. 1 shows an example of a silicon substrate with mask undercuts and notches. A substrate 100 may include a silicon base layer 102. An insulator layer 104 is disposed below a silicon layer 106, which may be disposed below a mask layer 108. To etch silicon layer 106, a fluorine-based gas may be used to form a trench 110. As silicon layer 106 is being etched, lateral etching may occur on sidewalls 112 and 114 of trench 110 causing mask undercuts 116 and 118.
In addition, when insulator layer 104 is reached, fluorine-based gases may cause deeper lateral etching of sidewalls 112 and 114 of trench 110 to create notches 120 and 122 into silicon layer 106. As mentioned above, fluorine-based gas mixtures that are employed to etch silicon are less effective etchants of insulator layers, which may cause the fluorine-based gases to etch more of the sidewalls 112 and 114 of trench 110, causing notches to be formed in silicon layer 106.
Mask undercuts and notches are undesirable because both mask undercutting and notches may cause unreliability or yield loss in the final product, such as a micro electro-mechanical system (MEMS) device. Some manufacturing companies have attempted to control the impact of mask undercuts by increasing the size of the mask. By empirically determining the size of a mask undercut, manufacturing companies may be able to compensate for the mask undercut by increasing the size of the mask, resulting in a more quality device. However, a larger mask usually results in fewer devices being formed from a substrate, thus an increase in cost.
Other manufacturing companies have attempted to control mask undercuts and notches by employing a low frequency plasma systems. Although, mask undercuts and notches may occur in both high and low frequency plasma processing systems, those skilled in the arts are aware that lateral etching component may be more difficult to control in a high frequency plasma system, resulting in more and/or deeper mask undercuts and notches. In some examples, if notches undercut too much of the silicon layer, other device features may be compromised. In an example, notches 124 and 126 have joined together to create a breakthrough 128, which may result in a defective device. Consequently, some manufacturing companies have sacrificed such benefits as faster etcher via a high frequency plasma system, in order to gain control of the lateral etching component, by reverting back to low frequency plasma systems.
Since silicon semiconductor industry is a highly competitive market, manufacturing companies are seeking more viable solutions for resolving the mask undercut and notching issues.